1. Free amino acid (faa) and humic substance (HS) concentrations were
measured seasonally using high performance liquid chromatography (HPLC) in two
contrasting freshwater ecosystems. FAA concentrations were generally higher in the
eutrophic Lewes Brooks than in the oligotrophic Isle of Thorns lake, whereas the
converse was the case with HS concentration.
2 Faa and HS accumulation patterns were measured in media conditioned by
Ceratophyllum demersum alone, Biompha/aria g/abrata alone, and media
conditioned by the snail and plant together. It was hypothesised that nutrient
exchange may benefit the modular components and that HS may also provide
benefits by acting as a growth factor or as a protective shield.
3 Total faa (tfaa) and HS accumulated at a significantly higher rate in axenic
compared to non-axenic Lemna cultures. This seems to be the first clear report of
HS formation without bacterial involvement. Tfaa concentrations accumulated at a
significantly higher rate in the axenic Lemna system, most probably due to the
absence of bacteria.
4 The rate of accumulation of exogenous faa in snail conditioned media (SCM)
decreased with temperature and oxygen availability, and displayed saturable
kinetics.
5 B. g/abrata tissue faa were shown to act as osmolytes. Thus their cellular faa
concentrations increased proportionately with rises in environmental osmolarity.
6 The exogenous faa medleys that accumulated in SCM were shown by
statistical methods to be species specific. These medleys may therefore serve as
sources of information and could have a role in chemotaxonomy.
7 B. g/abrata was capable of inward net accumulation of exogenous amino
acids, including a non-metabolisable amino acid analogue - aminoisobutyric acid
(AlB).
8 The experimental evidence suggested that AlB was accumulated by B.
g/abrata via an active amino acid transport system. The transport system had a low
Vmax (15.4 nmol/g/hr) and a Km of 20.6 f.lM, a concentration similar in magnitude to
that measured in natural sediments (chapter 2). Inward transport of exogenous faa
was calculated to meet 0.24 % of basal metabolism, and thus has a limited
nutritional role.